This disclosure relates generally to Tire Pressure Monitoring (TPM), more particularly to locating TPM wheel units with respect to a particular vehicle, and specifically to determining a wheel position of a tire pressure monitoring wheel unit on a vehicle, within the wheel unit, using loading information measured at the tire pressure monitoring wheel unit.
TPM systems may have been developed to monitor characteristics such as tire pressure and/or tire temperature of a vehicle and to report the characteristics to a receiver at a central monitoring station using radio transmissions. A monitor may be located adjacent each tire and periodically takes a measurement of the tire characteristics. The monitor may then transmits the results of the measurement in a radio frequency transmission to the central monitoring station which produces an alarm or a display in response to the measurement.
One issue in such systems has been the need to program the location of the transmitters at the vehicle's central receiver. To be fully useful, the tire characteristic data is preferably associated with the tire which originated the measurement when presenting a display or alarm. Each monitor typically can transmit unit identification information with the measurement. The tire monitor is preferably activated to produce this information and the information is then conveyed to the central receiver and associated with the position of the tire.
In direct TPM applications the sensor unit in the wheel has no way of making the determination if the wheel has been moved on the vehicle. Some systems have a method of transmitting right/left information which a least can alert the receiver as to which side of the vehicle the sensor is on.
Some of these existing solutions to provide such position information include Radio Frequency Detector (RFD) based systems. These systems employ one or more RFD's to determine which wheel a transmission originates from using received signal strength or other signal attributes. Problematically, such RFD-based systems require cables running from a central receiver to each RFD, typically four RFDs, each located in a wheel arch.
Front rear determination has sometimes been carried out by analyzing signal strength data at the receiver. Received Signal Strength Indicator (RSSI) based systems determine front and rear wheel positions by comparing received signal strength from each of the wheel units. However, this solution relies on the receiver being ‘biased’ to the front or rear of the vehicle in order to create a difference in received signal strength. Such systems are typically prone to RF power variations and can have problems with attaining a sufficient RF margin between front and rear.
LF (Low Frequency) triggers, typically coils in each wheel arch (or elsewhere nearby) may be employed to activate the wheel unit, which responds in a manner which indicates which LF trigger activated it, thus allowing the receiver to make the wheel position determination in some systems. These systems have similar problems to those encountered by RFD-based systems.
Many other TPM autolocation systems are typically power hungry in that the wheel units in such systems transmit more messages or data than typical, and/or include more than the typical components, straining battery resources within the wheel unit.